I have long had a great interest in the individuals who made such a great difference for God prepared their sermons. Accordingly, as I read the documents associated with various people, I always keep a lookout for key quotations or documents on this important task. God has blessed me to find many, and I will be slowly adding them here over time.—Dan
George Müller preached thousands of sermons at his own church and at many other venues world-wide when he made his many preaching tours towards the end of his life. He had somewhat accidentally discovered that preaching individual verses was the most powerful way that God seemed to bless. Accordingly he followed this approach in many of his sermons. However, he also spoke thematically. You won't waste your time reading about Müller's sermon preparation.
"In connection with the above, I must, however, state, that it appears to me there is a preparation for the public ministry of the Word, which is even more excellent than the one spoken of. It is this: to live in such constant and real communion with the Lord, and to be so habitually and frequently in meditation over the truth, that without the above effort, so to speak, we have obtained food for others, and know the mind of the Lord as to the subject or the portion of the Word on which we should speak. But this I have only in a small measure experienced, though I desire to be brought into such a state, that habitually "out of my belly may flow rivers of living water." (Read more on the subject by clicking on this link.)
J.C. Ryle's book Holiness is considered one of the best books ever written on the subject, and is still read with much interest in our day. The secret is the simplicity with which it is written, including short sentences, easy to understand words, etc. I have assigned this reading in the past to Seminary students and they have found it most helpful. After you read this, you should also read Holiness as an example of what he is speaking about.
"Whatever we preach, or whatever pulpit we occupy, whether we preach simply or not, whether we preach written or extempore, we ought to aim not merely at letting off fireworks, but at preaching that which will do lasting good to souls! Let us beware of fireworks in our preaching. “Beautiful” sermons, “brilliant” sermons, “clever” sermons, “popular” sermons, are often sermons which have no effect on the congregation, and do not draw men to Jesus Christ. Let us aim so to preach, that what we say may really come home to men’s minds and consciences and hearts, and make them think and consider." (Read the rest of Ryle's thoughts on Simplicity in Preaching.)